text: Many of the building blocks of biological systems such as sugars and amino acids are produced exclusively as one enantiomer. As a result, living systems possess a high degree of chemical chirality and will often react differently with the various enantiomers of a given compound. Examples of this selectivity include:
  * Flavour: the artificial sweetener aspartame has two enantiomers. -aspartame tastes sweet whereas -aspartame is tasteless.
  * Odor: R-(–)-carvone smells like spearmint whereas S-(+)-carvone smells like caraway.
  * Drug effectiveness: the antidepressant drug Citalopram is sold as a racemic mixture. However, studies have shown that only the (S)-(+) enantiomer is responsible for the drug's beneficial effects.
  * Drug safety: ‑penicillamine is used in chelation therapy and for the treatment of rheumatoid arthritis whereas ‑penicillamine is toxic as it inhibits the action of pyridoxine, an essential B vitamin.
  As such enantioselective synthesis is of great importance but it can also be difficult to achieve. Enantiomers possess identical enthalpies and entropies and hence should be produced in equal amounts by an undirected process – leading to a racemic mixture. Enantioselective synthesis can be achieved by using a chiral feature that favors the formation of one enantiomer over another through interactions at the transition state. This biasing is known as asymmetric induction and can involve chiral features in the substrate, reagent, catalyst, or environment and works by making the activation energy required to form one enantiomer lower than that of the opposing enantiomer.
  Enantioselectivity is usually determined by the relative rates of an enantiodifferentiating step—the point at which one reactant can become either of two enantiomeric products. The rate constant, k, for a reaction is function of the activation energy of the reaction, sometimes called the energy barrier, and is temperature-dependent. Using the Gibbs free energy of the energy barrier, ΔG*, means that the relative rates for opposing stereochemical outcomes at a given temperature, T, is:
  This temperature dependence means the rate difference, and therefore the enantioselectivity, is greater at lower temperatures. As a result, even small energy-barrier differences can lead to a noticeable effect.